Device for varying the inductance of a helical inductor

ABSTRACT

An insulating member has an external helical thread that threads inside the turns of a helical inductor. A first part of the thread provides a first spacing between turns of the inductor, and a second part of the thread provides a second, greater spacing between turns of the inductor. A nonmagnetic, conductive element is positioned inside the insulating member in the vicinity of the second part of the thread. With the first part of the thread engaging the inductor, a high value of inductance is provided. When the insulating member is rotated so that the second part of the thread engages the inductor, the increased spacing of the inductor turns and the nonmagnetic, conductive element combine to decrease the inductance of the inductor by an amount dependent upon the amount that the second part of the thread and the element are introduced into the inductor.

United States Patent Bunch et al.

[4 Mar. 7,1972

[54] DEVICE FOR VARYING THE INDUCTANCE OF A HELICAL INDUCTOR [72]Inventors: Raymond S. Bunch; Richard B. Sanger, both of Lynchburg, Va.

I 73] Assignee: General Electric Company [22] Filed: Oct. 12, 1970 [21]Appl. No.: 79,858

[52] 1.1.5. Cl. ..336/20, 336/75 [51] Int. Cl ..l-l01f2l/02 [58] Fieldof Search ..336/20, 75, 77, 136

[56] References Cited UNITED STATES PATENTS 2,458,071 1/1949 Gordon..336/20 2,438,359 3/1948 Clapp ..336/75 3,517,363 6/1970 Curtis et a1....336/136 2,251,631 8/1941 Mayer ..336/75 Primary Examiner-Thomas J.Kozma Attorney-James J. Williams, Frank L. Neuhauser, Oscar B. Waddelland Joseph B. Forman I [5 7] ABSTRACT An insulating member has anexternal helical thread that threads inside the turns of a helicalinductor. A first part of the thread provides a first spacing betweenturns of the inductor, and a second part of the thread provides asecond, greater spacing between turns of the inductor. A nonmagnetic,conductive element is positioned inside the insulating member in thevicinity of the second part of the thread. With the first part of thethread engaging the inductor, a high value of inductance is provided.When the insulating member is rotated so that the second part of thethread engages the inductor, the increased spacing of the inductor turnsand the nonmagnetic, conductive element combine to decrease theinductance of the inductor by an amount dependent upon the amount thatthe second part of the thread and the element are introduced into theinductor.

1 Claim, 4 Drawing Figures PATENTEDMAR 7 I972 RAYMOND 3. Emma, RICHARD8. SANGER,

BY fgfr zmmsm DEVICE FOR VARYING TI'IE INDUCTANCE OF A HELICAL INDUCTORBACKGROUND OF THE INVENTION Our invention relates to a device forvarying the inductance of a helical inductor,'and particularly to such adevice that is relatively simple in construction and efficient inoperation.

In many applications, particularly in radio transmitters, it isfrequently desirable to vary an inductor rather than a capacitor inorder to change circuit parameters or to change the resonant frequencyof a tuned circuit. The use of a variable inductor may often bedesirable in cases where the fixed, unavoidable capacities are too greatto permit addition of tuning capacity. In the case of some transmitters,the vacuum tube grid-cathode capacity was so large that any addedcapacity would have required an inductance that was to small to bepractical. In such applications, the inductor to be varied may utilizesolid copper wire wound in a self-supporting helix that has the desiredinductance, power, voltage, and mechanical characteristics. Magnetictuning slugs of powdered iron or ferrite materials have been provided tovary the inductance of such inductors. However, we have found that suchslugs introduce serious radiofrequency power losses, especially at thevery high radio frequencies, such as the 150 to 175 megaI-Iertz band ofinterest to us.

Accordingly, a general object of our invention is to provide a new andimproved device for varying the inductance of a helical inductor.

Another object of our invention is to provide an improved inductortuning device that is supported by the inductor without the need foradditional mechanical supporting structure.

Another object of our invention is to provide an improved inductortuning device that is capable of changing the magnitude of inductanceover a fairly wide range.

Another object of our invention is to provide a relatively simple buteffective device for varying the inductance of a helical inductor.

SUMMARY OF THE INVENTION Briefly, these and other objects are achievedin accordance with our invention by an insulated cylindrical device thathas a helical thread which fits inside the turns of a helical inductor.Part of the thread turns are spaced relatively close together, and partof the thread turns are spaced relatively far apart. When the inductorturns are engaged by the close thread turns, the inductor turns arefairly close together and the inductor provides a relatively highinductance. When the inductor turns are engaged by the far apart threadturns, the inductor turns are fairly far apart and the inductor providesa relatively low inductance. A nonmagnetic, conductive element,preferably in the form of a copper sleeve, may be concentricallypositioned along the longitudinal axis of the cylindrical device in thevicinity of the far apart thread turns so as to reduce the inductance ofthe inductor even more. Thus, we provide a device having a relativelysimple but effective structure for varying an inductor.

BRIEF DESCRIPTION OF THE DRAWING The subject matter which we regard asour invention is particularly pointed out and distinctly claimed in theclaims. The structure and operation of our invention, together withfurther objects and advantages, may be better understood from thefollowing description given in connection with the accompanying drawing,in which:

FIG. I shows an end view of a device in accordance with our inventionfor varying the inductance of a helical inductor;

FIG. 2 shows a side view of the device of FIG. 1;

FIG. 3 shows a cross-sectional view taken along the line 3 3 ofFIG. l;and

FIG. 4 shows a perspective view of a helical inductor provided with adevice in accordance with our invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIGS. 1, 2 and3, we have shown a preferred embodiment of a device 10 constructed inaccordance with our invention for varying the inductance of a spiral orhelical inductor. Our device 10 is generally cylindrical in shape, andis made of an insulating material such as a thermosetting plastic. Theexternal portion of the device 10 is provided with a continuous helicalthread 12 which has a constant diameter and which, for ease ofmanufacture, extends along the full length of the device 10. The thread12 is generally divided into two parts 12a, 12c. The first part 12aextends from one end to the approximate midpoint, and has thread turnswhich are spaced relatively close together, and which are shaped so asto engage the internal surface of the wire forming the inductive helixwhich is to be tuned. At approximately the midpoint of the device 10, atransition 12b is provided between the first part 12a and thesecond part12c The second part 12c extends from the transition 12b to the otherend, and has thread turns which are spaced relatively far apart andwhich are also shaped to engage the internal surface of the wire formingthe inductive helix. Thus, the pitch of the second part is greater thanthe pitch of the first part 12a. As is known, a nonmagnetic, conductivesleeve or cylinder 14, preferably made of copper or brass and silverplated to keep losses low, may be concentrically positioned inside thedevice to extend along the longitudinal axis between the midpoint andthe other end. At this other end, a recess 16 and a slot 17 (or hole)may be provided for receiving a screwdriver blade (or tool) so that thedevice 10 can be rotated. We have found that present molding techniquesfor thermosetting plastics permit the sleeve 14 to be positioned in amold and the entire device, along with thread 12 and the recess 16 andslot 17, formed in a single molding operation.

FIG. 4 shows a perspective view of a spiral or helical inductor 20 witha device 10 in accordance with our invention. The device 10 and theinductor 20 are constructed so that the device 10 can be threaded insidethe inductor 20 with the first part 12a of the thread 12 normallyengaging the turns of the inductor 20. In this connection, it will beseen that the shape of the threads 12 of the device 10 corresponds tothe shape of the wire or material forming the turns of the inductor 20so that a threading match or engagement can be provided. This ensuresthat the device 10 is self-supporting within the turns of the inductor20. With the device 10 in this position, the inductor 20 has its maximumor largest value of inductance. If the inductance is to be varied, thedevice 10 is turned or rotated (in a clockwise direction as viewed inFIG. 4) so that the second part 120 of the thread 12 moves into theinductor 20. As this takes place, the greater spacing of the threadturns forming the second part 12c causes the turns of the inductor 20 tospread apart in an axial direction, and thus reduces the inductance ofthe inductor 20. In addition, the copper sleeve 14 adds an eddy currentflow so that the inductance is further reduced. The reduction ofinductance can be varied to a fine degree, since a relatively smallchange in inductance is provided by a half or full turn of the device10. However, the relatively large number of turns of the thread 12permit the inductance to be varied over a relatively wide range.

In one embodiment of our device 10 which was constructed, the device hada length of approximately 1 inch and an external diameter ofapproximately one-half inch. The device fitted into a wound inductorwhich had approximately three turns and which was connected with acapacitor to provide a resonant frequency of approximately megaHertz.When the device was threaded into the inductor so that the turns werespaced farther apart and the copper sleeve 14 was within the inductor,the inductance was reduced so that the resonant frequency was increasedto approximately megaHertz.

It will thus be seen that our invention provides a new and improveddevice which, although relatively simple, is effective in varying theinductance of a helical inductor. While we have shown only oneembodiment, persons skilled in the art will appreciate thatmodifications can be made. For example, the physical dimensions andpitch of the thread parts having respective fixed pitches, a thread ofconstantly varying pitch may be provided. Such a thread would provide afiner control of inductance variation. The thickness and length of thecopper sleeve may be varied. A magnetic core may be positioned in thedevice in the vicinity of the relatively closespaced threads (eitherwith or without the nonmagnetic conductive sleeve) to provide an addedinductance variation. Therefore, while our invention has been describedwith reference to a particular embodiment, it is to be understood thatmodifications may be made without departing from the spirit of theinvention or from the scope of the claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An improved device for va ying the inductance of a cylindricallyshaped wound coil having a plurality of turns of selected diameter andlength, said device comprising:

a. an elongated cylindrical member of insulating material;

b. said member having first external threads which extend from one endto an intermediate point thereof for engaging the internal part of turnsof a wound coil;

.c. said member having second external threads which extend from saidfirst threads and said intermediate point to the other end thereof formengaging the internal part of turns of a wound coil;

d. said first threads having a first selected pitch and said secondthreads having a second selected pitch that is greater than said firstpitch and that is joined thereto by a smooth transition so as to spacethe turns of a coil by said second threads and thereby change theinductance of a wound inductor;

e. and a nonmagnetic, conductive element symmetrically positioned aboutthe longitudinal axis of said cylindrical member and within saidcylindrical member in the vicinity of said second threads, for providingan added effect on the coil inductance.

R UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,648,205 Dated March 7, 19 72 t Raymond S. Bunch, Richard B. Sanger Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 3, line 3 after "thread" insert can be varied.

Instead of a transition between two thread 4 Column 4, line 6 cancel"form" and insert for Signed and sealed this 2 +th day of April 1973.

(SEAL) Attest:

EDWARD M. FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM (10-59) USCOMM-DC scan-P69 U.5. GOVERNMENT PRINTINGOFFICE: [9G9 D365334 Patent No. 3,648,205 Dated March 7, 19 72 I t r(Raymond S. Bunch, Richard B. Sanger It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, line 3 after "thread" insert can be varied.

Instead of a transition between two thread Column 4, line 6 cancel"form" and insert for Signed and sealed this 2mm day of April 1973.,

v (SEAL) Attest:

EDWARD M. FLETCHER, J'R. ROBERT GOTTSCHALK Attesting OfficerCommissioner of Patents FORM PO-IO50 (10-69) sco -pc 5 37e.p59

w u.s, GOVERNMENT PRINTING OFFICE: I969 o-sss-asa

1. An improved device for varying the inductance of a cylindricallyshaped wound coil having a plurality of turns of selected diameter andlength, said device comprising: a. an elongated cylindrical member ofinsulating material; b. said member having first external threads whichextend from one end to an intermediate point thereof for engaging theinternal part of turns of a wound coil; c. said member having secondexternal threads which extend from said first threads and saidintermediate point to the other end thereof form engaging the internalpart of turns of a wound coil; d. said first threads having a firstselected pitch and said second threads having a second selected pitchthat is greater than said first pitch and that is joined thereto by asmooth transition so as to space the turns of a coil by said secondthreads and thereby change the inductance of a wound inductor; e. and anonmagnetic, conductive element symmetrically positioned about thelongitudinal axis of said cylindrical member and within said cylindricalmember in the vicinity of said second threads, for providing an addedeffect on the coil inductance.